This invention relates to image processing apparatus and methods for processing digital colour data representing a colour or black-and-white image to enhance the rendition of text in presentations of said image, and in particular, but not exclusively, to such apparatus and methods for use in a colour scanner printer, copier, facsimile or the like.
In many existing designs of colour copier, the original image is scanned and digitised to provide red, green and blue (RGB) digital signals representing the relative intensities of these three colours at each pixel of the scanned image. The RGB data is converted to cyan, magenta, yellow and black (CMYK) signals by a colour processing pipeline, and the CMYK signals are used to control the amount of cyan, magenta, yellow and black ink or toner deposited at the appropriate pixel in the printed image.
Whilst the colour processing pipeline is able to deal appropriately with continuous tone colour image data, it does not perform well with areas of text, i.e. the processing of simple text on a white or plain coloured background. Due to a mixture of scanner inaccuracies and the non-linearities in the colour mapping between RGB and CMYK, which are desirable for continuous tone and to push near white background regions to white, the text tends to be rendered both too light and with too much colour in and around it. The adverse effects on coloured text or black text on coloured background are perceived as being less critical. Similar problems arise in equipment other than copiers, for example laser or ink-jet colour printers, colour display units, colour scanners and the like, where the image is represented as a collection of chromatic signals.
U.S. Pat. No. 5,189,523 discloses a system in which an image is scanned in a first scan to provide a discrimination signal, for example, using an edge extraction filter and a "black judgement" step. The discrimination data is stored in a bit map memory which is then used during a second scan to control whether data from the second scan is enhanced or used raw.
European Published Application No. 463,844 discloses a system in which an input image is processed to enhance the boundary region between a text letter or item and the background. Both these earlier proposals concentrate on the edge or interface between black and white; they do not take into account further features within the image components or connectivity, and so in some instances may highlight the edge of a text glyph whilst degrading its overall form.